Fix CI workflow and README

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

.github/workflows/ci.yml

@@ -92,7 +92,7 @@ jobs:
             localhost:5000/name/app:latest
             localhost:5000/name/app:1.0.0
           secrets: |
-            GIT_AUTH_TOKEN=01234567890abcdefgh
+            GIT_AUTH_TOKEN=${{ github.token }}
       -
         name: Inspect
         run: |

README.md

@@ -80,8 +80,9 @@ jobs:
         run: echo ${{ steps.docker_build.outputs.digest }}
 ```
 
-If you want to authenticate against a private repository, you have to use a secret named `GIT_AUTH_TOKEN` to be able
-to authenticate against it with buildx:
+Building from current repository automatically uses the [GitHub Token](https://help.github.com/en/actions/configuring-and-managing-workflows/authenticating-with-the-github_token)
+as provided by `secrets` so it does not need to be passed. But if you want to authenticate against another private
+repository, you have to use a secret named `GIT_AUTH_TOKEN` to be able to authenticate against it with buildx:
 
 ```yaml
       -